1. Field of the Invention
The present invention relates to a catalyst feeding system for polymerization proceses and, more particularly, to such a system in which catalyst is mixed with a hydrocarbon fluid in a tank to form a catalyst mud prior to introduction into the polymerization system.
2. Setting of the Invention
In some vapor phase or slurry phase olefin polymerizations a polymerization catalyst, such as CrO.sub.3 -SiO.sub.2, CrO.sub.3 -SiO.sub.2 -Al.sub.2 O.sub.3, TiCl.sub.3, TiCl.sub.3 -MgCl.sub.2 and the like, is usually introduced at timed intervals into one or more polymerization zones within a reactor vessel. In the reactor vessel the catalyst and a quench liquid, such as a light hydrocarbon, are introduced directly into and onto the stirred and/or fluidized bed for forming the desired polymers. In one type of polymerization system the catalyst and a hydrocarbon, such as hexane for propylene polymerization or isobutane for ethylene polymerization, are introduced into a mixing tank, which can be equipped with rotating mixing blades. The catalyst and the hydrocarbon are mixed at a high rate of speed to form a fluid-like catalyst slurry which is then introduced through a ball or shot valve into a catalyst dilution vessel and then into the reactor vessel. If the mixing speed is too slow the catalyst can become compacted in the lower portion of the tank and often will not flow into the shot valve. If the mixing speed is too fast the catalyst may be pulverized, which then results in the production of undesirable polymer fines in the reactor vessel. Also, in these systems the catalyst slurry may not be introduced into the dilution tank or the reactor vessel in exact amounts each time because the catalyst slurry tends to not feed into the ball or shot valve in a predictable manner.
In certain situations, it is desirable to react the catalyst with a suitable olefin containing hydrocarbon to form a catalyst-polymer-curd for introduction into the reactor vessel. The curd makes a higher activity catalyst because of better promoter-catalyst contact in the presence of liquid phase olefins. Also, temperature control in the reactor vessel is better because the catalyst is bathed in hydrocarbon when it is initially formed and is in a high activity state. To produce the catalyst-polymer-curd, additional equipment is usually required to be installed in the catalyst feed system, which then adds to the installation cost and maintenance expenses of the total system.
There exists a need for a polymerization catalyst feed system which (a) reduces or eliminates pulverization of the catalyst, (b) can accurately measure and feed a desired amount of catalyst into a dilution tank or the reactor vessel, and (c) has the flexibility to form a catalyst-polymer-curd without the need for additional equipment.